This invention concerns pattern-forming materials, or resist, used in the manufacture of, for example, semiconductor devices, and in particular a suitable pattern-forming material which can be used as a resist sensitive to deep UV radiation of wavelength 300 nm or less, to electron beams, to x-rays, and the like.
With increasing levels of integration and speed in semiconductor devices such as integrated circuits, demands required for the microfabrication of semiconductors are becoming more and more stringent. Resistant process technology is fundamental to the manufacture of semiconductors, and techniues for obtaining fine patterns or any other desirable patterns are therefore necessary.
Various substances have been used as pattern-forming materials in this process (sometimes referred to hereafter as resists). Owing to their high degree of resolution, positive resists consisting of a Novolac resin base with addition of naphthoquinone diazide photosensitive agents were often employed. With this positive resist and a reduction projection exposure machine using the g line from a super-high pressure Hg lamp (wavelength 436 nm), it is possible to obtain a resist pattern with a line resolution width of about 0.8 .mu.m.
The resolution in the imaging space is expressed in the equation: EQU R=k.lambda./NA
where .lambda. is the wavelength of the light used for the exposure, and NA is the numerical aperture of the objective lens. To improve R still further, therefore, it is necessary either to reduce .lambda. or to increase NA.
If however NA is increased, there is a problem in that the focal depth becomes shallower. Moreover, to maintain a certain field size, a wide diameter lens has to be used, but it is often difficult to manufacture such a lens. It is thus undesirable to increase NA.
The wavelength of the light source, .lambda., on the other hand, can be reduced by using an Xe-Hg lamp or a KrF excimer laser (wavelength 249 nm) which has now come into use as a light source for lithography. It is thus possible to obtain a finer resist pattern by using these light sources and resists that are suited to them.
The following materials are known to be suitable as resists for use with short wavelengths.
For deep UV radiation, there are both positive and negative resists.
An example of a positive resist is MP2400 manufactured by Shipley. This resist absorbs deep UV radiation of wavelength 200-300 nm very well and since it is a positive resist, the part of the resist layer that is to be removed by development has to be fully exposed in order to obtain the pattern. The result is that the upper part of the resist layer to be removed is over-exposed, and the resulting resist pattern after development has a trapezoid shape.
With such a trapezoid resist pattern, as the resist film is thinner towards the fringe of the trapezoid, it is easily removed by etching. For this reason, it is not possible to carry out submicron rule processing of the pattern with the required accuracy.
An example of a negative deep UV resist is the poly (vinylphenol) and bis-azide composition disclosed in IEEE Transactions on Electron Devices, ED-28 (11), November (1981), p. 1306.
As the resist in this reference strongly absorbs deep UV light, the part of the resist rendered insoluble in the developer is limited to the upper part of the layer. The insoluble part is then used as a mask for development, and it is reported that by controlling the developing time, the profile of the remaining resist layer can be changed. Using this resist and contact printer with an Xe-Hg lamp and cold mirror CM-290, it was possible to resolve 0.4 .mu.m line with 1.6 .mu.m pitch. As we have stated, the negative resist disclosed in this reference has a strong deep UV absorption of more than 2 .mu.m.sup.-1, and using this property, it is possible to render only the upper part of the layer insoluble in developer after it has been exposed. The pattern dimensions are determined by the insoluble part. The part of the resist which lies directly underneath the exposed area, and which is only slightly exposed, was then developed isotropically to give the resist pattern.
When however it was attempted to form a resist pattern with, for example a line width at sub-micron rule and having an aspect ratio of 2 or more, the part slightly exposed directly beneath the insoluble part dissolved during the development. It was therefore difficult to obtain a resist pattern with a high aspect ratio.
In addition, for an Xe-Hg lamp of high photometric output and light of shorter wavelength in the region of 250 nm, the sensitivity of this negative resist drops and so full advantage could not be taken of short wave light sources.